Johannes Kirchhof

Efficient Yb laser fibers with low photodarkening by optimization of the core composition

We report on photodarkening (PD) investigations at Yb doped fibers with specific variation of the concentrations of the codopants aluminum and phosphorus, measured during cladding pumping at 915 nm. A core composition with equal content of Al and P is most promising to achieve Yb fibers with low PD, high laser efficiency and low numerical aperture of the laser core despite of high codoping. A laser output power of more than 100 W was demonstrated on such a fiber with a slope efficiency of 72%. The correlation of the PD loss with the NIR-excited cooperative luminescence encourages the supposition that cooperative energy transfer from excited Yb(3+) ions to the atomic defect precursors in the core glass enables the formation of color centers in the pump-induced PD process.

Photodarkening in Yb doped fibers: experimental evidence of equilibrium states depending on the pump power

Photodarkening in Yb doped fibers was examined at 633 nm in-situ during cladding pumping at 915 nm with varying pump powers and with no indication of an onset threshold. For the first time, the partial bleaching of the photodarkening loss by the pump power itself was observed. We found the relaxation to well-defined equilibrium states of the core excess loss, depending on the Yb inversion. From the dependence of the measured rate constant on the density of excited Yb ions we conclude, that on average 3 to 4 excited Yb ions create or bleach one color center responsible for the core excess loss.

High-power cw neodymium-doped fiber laser operating at 9.2 W with high beam quality

We report on a 9.2-W cw, two-times diffraction-limited, neodymium-doped fiber laser operating in the 1.06-microm region. For a silica-based double-clad fiber, slope efficiencies of more than 25% are observed for pumping by a diode-laser system operating near 810 nm.

Multifilament-core fibers for high energy pulse amplification at 1.5 μm with excellent beam quality

We report on what we believe to be the first demonstration of an erbium-ytterbium-doped multifilament-core (MFC) fiber for single-mode amplification of narrow linewidth high peak power pulses. A master-oscillator-power-fiber-amplifier laser source has been demonstrated using a 37-filament MFC fiber in the last amplification stage. Pulses with 750 microJ (940 W peak power) and laser linewidth<1 MHz have beam generated with M2 approximately 1.3. This value is close to the theoretical value M2 approximately 1.5.

A new material for high-power laser fibers

We have developed a new technique to produce a Yb-doped fused silica bulk glass which is very well suited for fiber laser applications. The starting point is a liquid suspension of SiO2 particles which is doped by a solution of rare earth ions. After dehydration, purification and vitrification we achieve a bubble-free homogeneous Yb-doped fused bulk silica, which is further processed by the plasma outside deposition (POD) technique into preforms for active laser fibers with a large active fiber core. The laser function of our Yb-doped silica was successfully proved in a side-pumped fiber laser setup. We present the results of the laser experiments.

Photodarkening kinetics as a function of Yb concentration and the role of Al codoping

We investigated the photodarkening (PD) kinetics of two fiber series with variation of the Yb content for constant Al concentration or constant ratio of Al/Yb, respectively. The results show the outstanding importance of the absolute value of Al concentration also in the case of fibers with strongly reduced Yb content. An Al/Yb ratio of 5 to 6 is not sufficient to mitigate PD loss. Moreover, a model to describe PD loss and rate constant as functions of Yb concentration and excitation is suggested that links measurements of PD in single fibers of the same type (variation of Yb inversion) and in fiber series (constant Yb inversion).

Non-isothermal bleaching of photodarkened Yb-doped fibers.

We report on the thermal treatment of photodarkened Yb-doped fiber samples. The method of non-isothermal bleaching at different temperature ramp rates can be used to determine the thermal energy distribution of photodarkening induced color centers. A distributed activation energy with a mean value of about 1.3 eV and a FWHM of 0.5 eV was found. Spectral changes during thermal treatment were observed and could be interpreted, e.g. as an enhancement of the absorption cross section.

Dopant interactions in high-power laser fibers

Rare earth doped silica based fiber lasers and amplifiers with very high output power and excellent beam quality are efficient devices for a variety of applications in industry, science and medicine. During the last years, important progress was possible by new design concepts but also by carefully tailoring the material properties. Aspects of material and technology development concerning the interaction of different dopants and co-dopants will be discussed in the following. Our discussion concentrates on the optical properties of the laser fiber as refractive index, absorption and emission of the rare earths and especially on phenomena concerning the background loss of the fibers. We have found a strong rare earth specific loss component which remarkably depends on the kind and the ratio of the co-dopants. The relations of this background loss to material composition and fabrication technology are demonstrated and discussed on an empirical base.

Influence of Tm- or Er-codoping on the photodarkening kinetics in Yb fibers

We investigated photodarkening (PD) parameters of Yb/Al-doped silica fibers as a function of the concentration of additional rare earth ions like Tm or Er. It was found that both Tm and Er cause a decrease in Yb inversion followed by a reduction of PD in the case of Er, whereas Tm-codoping with more than 10 mol-ppm can strongly accelerate the process and also increase the PD loss. However, contrary to [1], we conclude that the typical PD behavior of Yb/Al fibers is an intrinsic feature of this fiber type and not caused by trace impurities of Tm (< 1 mol-ppm) unintentionally incorporated by the raw materials during fiber preparation.

Codoped materials for high power fiber lasers - diffusion behaviour and optical properties

Optical fibers for high power lasers and amplifiers are fabricated on the basis of quartz glass which has outstanding properties concerning high fiber strength, high power hardness and low optical losses compared with other glasses such as heavy metal fluoride or oxide glasses. It is well known, however, that the host properties of pure silica regarding the active rare earth ions are insufficient and the laser medium has to be improved by the incorporation of codopants. Here we present new investigations of material and fiber properties for phosphorus/aluminium codoping, with regard to the realization of efficient rare earth doped cw and pulsed high power fiber devices. The diffusion behaviour in the complex systems shows characteristic interaction effects, which influence the dopant concentration and their spatial distribution. The refractive index in the codoped systems and the basic attenuation deviate remarkably from additivity relations. The absorption spectrum in the VIS/NIR region depends on codopant concentration and on preparation conditions, with influence on the fluorescence properties of the rare earths and the laser efficiency.